Clamp-on transformer



Nov. 17, 1959 O. C. ENIKEIEFF CLAMP-0N TRANSFORMER Filed June 12, 1958 INVENTOR ATTORNEYS United States Patent CLAlVfP-ON TRANSFORMER Oleg C. Enikeietf, Silver Spring, Md., assignor to Harry C. Miller Company, Rochester, N.Y., a partnership Application June 12, 1958, Serial No. 741,658

4 Claims. (Cl. 336-175) The present invention relates in general to electric inductive coupling devices, and more particularly to clamp-on inductive transformers designed to inductively couple intelligence bearing signals from a conductor to be amplified and received at a listening or detecting station.

Heretofore, numerous clamp-on type electrical measuring instruments have been devised and made commercially available for clamping on to conductors to facilitate measurement of very large 60 cycle alternating current power currents. These are very large, bulky instruments commonly known as through-type ammeters, and the basis of design of these commercially available ammeters is such that they do not lend themselves to miniaturization or application to the measurement of higher frequency currents. The basic mechanical features, for example, of the commercial through-type ammeters are such that they are not amenable to miniaturization. From the standpoint of electrical properties, these commercially available through-type ammeters utilize pickup coils which are only sensitive to very strong signals. If these commercial designs were made more sensitive by increasing the number of turns in the pickup coils, the leakage flux would become so high that the devices would lose their capability to discriminate between the signal on the specific wire conductor which they were monitoring and external magnetic disturbances.

Several varieties of devices for inductively picking up audio signals in telephone communication systems have been designed but these consistently take the form of inductive pads designed to be positioned beneath a telephone receiver stand or desk set or to be inserted in the bell box of a Wall telephone immediately adjacent the regular induction coil of the telephone. This latter type of pickup device obviously requires unauthorized and surreptitious entry into the telephone bell box and alteration of the internal structure of the bell box, and

. the former system requires access to the telephone desk set.

An object of the present invention is the provision of a very compact clamp-on transformer of miniature design and construction which may be readily clamped on a current carrying conductor to pick up audio, ultrasonic and radio frequency currents in the conductor and permit amplification and detection thereof.

Another object of the present invention is the provision of a novel miniature clamp-on type transformer which can be used to qualitatively and quantitatively measure audio, ultrasonic and radio frequency currents in single insulated conductors without making any direct electrical connections.

Another object of the present invention is the provision of a miniature clamp-on transformer for inductively coupling signal currents from a conductor in a simple and convenient manner without making a direct electrical connection, which may be conveniently coupled to the conductor at any point where access to a small segment of the conductor is available, and which is of very simple and convenient construction.

Another object of the present invention is the provision of a novel miniature clamp-on transformer of the above character which is sensitive to currents of the order of several micro amperes and discriminates very strongly against magnetic disturbances produced by external sources and against currents carried in adjoining exterior conductors relative to the conductor about which it is clamped.

Another object of the present invention is the provision of a novel miniature clamp-on transformer for monitoring audio, ultrasonic, and radio frequency signal currents, which has the ability to demodulate ultrasonic and radio frequency carriers.

Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description taken in conjunction with the attached drawing illustrating one preferred embodiment of the invention.

In the drawing:

Figure 1 is a schematic perspective view of a clamp-on type transformer embodying the present invention illustrated in a condition of use in conjunction with a miniature audio amplifier and an ear phone head set;

Fig. 2 is a vertical transverse section view of the clampon transformer, with one of the shell halves removed to reveal the interior of the transformer;

Fig. 3 is a horizontal transverse section view taken along the line 3-3 of Fig. 2;

I permit the amplified output signal to be Fig. 4 is a vertical section view taken along the line 4-4 of Fig. 2.

Referring to the drawing wherein like reference characters designate corresponding parts throughout the several figures, and particularly to Fig. 1 illustrating the transformer in a condition of use, the clamp-on transformer 10 is clamped on to one insulated conductor 11 of a two wire set of conductors designated generally by the reference character 12, so as to inductively pick up currents in the conductor 11. The transformer 10 may be conveniently coupled to a miniature audio amplifier 13 by means of a line 14 having a suitable matching transformer 15 therein and connectors such as plugs at each end. The miniature audio amplifier 13 may be of any suitable pocket size or readily portable type designed to amplify signals over the frequency range of signals to be monitored by the transformer. The audio amplifier 13 will preferably have an outlet jack or plug socket to coupled to a suitable transducer device such as an ear phone head set 16. By means of these components, the clamp-on audio transformer may be conveniently used to monitor telephone conversations by clamping the transformer on to a telephone system conductor at any place where a small segment of the conductor is available. This provides a highly convenient portable facility to permit monitoring of telephone conversations or qualitative or quantitative measurements of signal currents for any desired purposes.

The clamp-on transformer 10 comprises a pair of shell halves 17, 18 molded from plastic material such as nylon, designed to be mated together to form a casing 19 for enclosing and supporting the components of the transformer. The shell halves 17, 18 are formed with an enlarged lower chamber 20 and an upper chamber 21 near the upper end of the casing 19 opening through the upper wall of the shell halves which communicates with the larger lower chamber 20. Supported within the chamber 21 at the upper portion thereof is a central core block 22 and a pair of side core blocks 23 and 24 flanking the central core block 22. The central core block 22 is of square cross section and is provided with a pair of longitudinally spaced, parallel circular grooves formed in its periphery, as indicated at 25, dividing the block 22 into three fin-like formations 26 of rectangular cross section spaced by the two cylindrical grooves 25.

as aess V a in The side core blocks 23 and 24 have identical formations at the lower portion thereof, which are designated by the reference 25 and 26' corresponding to the grooves 25 and fins 26, respectively, and upwardly projecting cylindrical sections 27 provide forms about which the transformer windings 28 and 29 are wound.

The circular core block 22 and side core blocks 23 and 24 are supported in the shell halves 17, 18 by rib formations indicated at St? at the opposite ends of the upper chamber 21 which complement the outline of the grooves 25 in the side core blocks 23, 2d and project into them, and a pair of ribs 31 which project into the chamber 21 from the sides of the chamberand are aligned with tr e lower grooves only of the grooves 25 and '25 of the middle and side core blocks.

The middle and side core blocks 22, 23 and 24 are further held together within the'shell halves 17, 18 by a pair of clamp blades 32 which are substantially complementary in cross section to the elevational outline of the grooves 25, 25 to be tightly seated into the upper grooves- 25, 25 of the core blocks, the length of the clamp blades being approximately equal to the length of the ribs 31.

The upper cylindrical sections 27 of the side core blocks 23, 24 terminate in square cross section heads 33 and the right hand side core block 24, as viewed in Fig. 2, is provided with a hollow central bore 34 adapted to slidably receive an elongated core stem 35 of circular cross section which projects above and below the side core block 24. The upper end of the core stem 35 is threaded into and supports a movable core bridging member 36 which spans the distance between the two side core blocks 25, 24 and normally rests upon the fiat upper surfaces of the heads 33 of the side core blocks in the solid line position illustrated in Fig. 2. The lower end of the core stem 35 projects into the enlarged chamber 26 and has secured to the lower end thereof a guide plate 37 which is of generally rectangular configuration but has a laterally projecting tongue 38 substantially complementing and projecting into a guide groove formed by two corner bosses 39 provided on the shell halves 17, 18 to confine the plate 37 to the position illustrated in Fig. 3 and effectively center the same when it occupies the lower limit of its range of movement. A compression spring 49 bearingat one end on the guide plate 37 and at its upper end on the shoulder 41 connecting the end wall of the chamber 21 with the end wall of the chamber 26 to continuously urge the guide plate 37 and the core stem 35 and core bridging member 36 secured thereto downwardly to the solid line position shown in Pig. 2. A push button 42 having a central cylindrical section terminating in flanges at the opposite ends thereof is seated in a complementary aperture 43 of circuiar configuration in the lower wall of the shell halves =17, 18, formed by semicircular openings in the mating shell halves. The push button 42 is supported for axial movement inwardly of the casing 19 by the aperture 43 so that upward pressure upon the push button 42 sufilcient to overcome the force of the spring will effect axial displacement of the core stem 35 upwardly within the bore of the side core block 24 to permit movement of the core bridging member 36 to the dotted line position illustrated in Fig. 2 wherein it is spaced a suficient distance from the head 33 of the side core block 23 to admit an insulated conductor between these elements and into the space between the windings 28, 29 on the cylindrical sections 27 of the side core blocks 23, 24.

The central core block 22 is also provided with a longitudinal bore through its center for receiving a length of insulating tubing 44 which may be formed of a flexible plastic material extending entirely through the central core block 22 and depending some distance below it through which the wire ends or leads 28', 29' of the windings 28, 29 extend to the terminal members 45,

46 of a jack fitting 47 of conventional construction associated with the base of the casing 19. The jack fitting 47 has a centrally apertured threaded shank 43 extending through an opening 4? formed of semi-circular openings in the matin' shell halves l7, 1%, the jack fitting 47 being fixed to the casing l9 by a washer and nut assembly 5i) threadedly associated with the shank 48 and bear'ng against the exterior surface of the lower wall of the casing 15 The central opening in the shank 48 is designed to receive a plug of conventional construction on the end of a multi-wire conductor, for example the conductor lid, to electrically connect the winding leads 28, 29' to the wires of the conductor 14 and lead the signal to an amplifier unit such as the portable amplifier 13.

The shell halves 17, 13 are removably secured together in properly registered and mated relation, as illustrated in Pig. 4, in a conventional manner as by means of a bolt 51 extending through one of the shell halves and a nut 52 having a threaded socket for receiving the threaded shank of the bolt 51 and having a knurled periphery as indicated at 53 to bite into the plastic material of the shell half 18 and hold the nut 52 against rotation. V

The windings 28, 29 in the preferred example herein described each comprise 64 turns of wire of about No. 34 A'.W.G. although variations in the gauge and number of turnsof the wire formingthe windings 28, 29 may be varied to adapt the transformer to various specific applications as will be well understood by persons skilled in the art.

7 It will be apparent that the core of the transformer is efiectively formed as a substantially closed loop core comprising the core bridging member 36, the side core blocks 23, 24, the central core block 22 and the core stem 35, together with the clamp blades 32 interconnecting and forming a high permeability flux path between the side core blocks 23, 24 and the central core block 22.

Each of these elements, namely, the core bridging member 36, the side core blocks 23, 2 the central core block 22 and the core stem 35 are preferably formed of Allegheny-Ludtum relay steel number 2 annealed in a hydrogen atmosphere. The selection of this core material has been found to be highly desirable both from the point of view of the high sensitivity and discriminating characteristics of the transformer, and to effect demodulation of ultra-sonic and radio frequency carriers when the clamp-on type transformer is used to monitor ultra-sonic and radio freq enc I signals. The demodulation takes place as a result of non-linearity of the core material. When the clamp'on transformer is used for this purpose at the ultra-sonic or radio frequencies, the impedance of the device increases proportionately with the frequency, and the use of a matching transformer such as the matching transformer 15 is not necessary. The non-linearity of the field intensity H and flux density B relationship or magnetization curve, the field intensity being a product of the current in the monitored conductor and the flux density a product of the core material roperties, produces a non-linear system which results in the demodulation.

The output impedance of this clamp-on type transformer is purposely kept low to cut down on the number of turns and to achieve low leakage flux. The result is the production of a highly compact miniature clamp-on type transformer which is sensitive to currents of the order of several micro amperes and discriminates very strongly against magnetic disturbances produced by external sources. These'desirable properties are achieved by the conjunction of the novel geometric design, the reduction in the number of coil turns, and the selection of Allegheny-Ludlum relay steel number 2 annealed in a hydrogen atmosphere as an optimum compromise core material. This however, is not to exclude the possibility of using the other core materials which-would have similar characteristics.

By the use of a well shielded external matching transformer such as the matching transformer between the clamp-on transformer 15 and the amplifier 13, the response of the device extends from cycles through the lower radio frequency range.

In the above description, the invention has been disclosed merely by way of example and in preferred manner, but obviously many variations and modifications may be made therein as will be apparent to persons skilled in the art. It is to be understood, therefore, that the invention is not limited to any specific form or manner of practicing the same except in so far as limitations are specified in the appended claims.

What is claimed is:

l. A clamp-on transformer for picking up currents from an insulated conductor to monitor signals therein comprising a casing of insulating material, a magnetic circuit supported by said casing including a pair of laterally spaced elongated side core pieces disposed along generally parallel axes with inner end portions within said casing and outer end portions exposed externally of said casing and a connecting core piece within said casing forming a flux path between the inner end portions of the side core pieces and a bridging core member adapted to abut the ends of said outer end portions of said side core pieces, said side core pieces, connecting core piece and bridging core member forming a generally rectangular closed loop of magnetic core material, an elongated stem of core material fixed at one end to said bridging core member and extending into said casing through one of said side core pieces in slidably supported relation for axial reciprocation from an inner position wherein said bridging core member contacts both of said side core pieces to an outer position wherein the bridging core member is spaced from the end of the other of said side core pieces to accommodate passage of the insulated conductor therebetween, pickup coils on the exposed outer end portions of said side core pieces disposed to be in inductive communication with currents in the insulated conductor when the conductor is positioned therebetween, and coupling means supported by said casing in electrically conductive communication with said coils for conducting to an external facility currents conductively coupled into the coils from the insulated conductor.

2. A clamp-on transformer for picking up currents from an insulated conductor to monitor signals therein comprising a casing of insulating material, a magnetic circuit supported by said casing including a pair of laterally spaced elongated side core pieces disposed along generally parallel axes with inner end portions within said casing and outer end portions exposed externally of said casing and a connecting core piece within said casing forming a flux path between the inner end portions of the side core pieces and a bridging core member adapted to abut the ends of said outer end portions of said side core pieces, said side core pieces, connecting core piece and bridging core member forming a generally rectangular closed loop of magnetic core material, an elongated stem of core material fixed at one end to said bridging core member and extending into said casing through one of said side core pieces in slidably supported relation for axial reciprocation from an inner position wherein said bridging core member contacts both of said side core pieces to an outer position wherein the bridging core member is spaced from the end of the other of said side core pieces to accommodate passage of the insulated conductor therebetween, resilient means within said casing bearing against said stem for biasing the stem and bridging core member to said inner position, a push button slidably supported in said casing for manually forcing said stem and bridging core member against the biasing action of said resilient means to said outer position, pickup coils on the exposed outer end portions of said side core pieces disposed to be in inductive communication with currents in the insulated conductor when the conductor is positioned therebetween, and coupling means supported by said casing in electrically conductive communication with said coils for conducting to an external facility currents conductively coupled into the coils from the insulated conductor.

3. In a clamp-on transformer for inductively picking up currents from an insulated conductor to monitor signals therein, the combination recited in claim 2, wherein said side core pieces are formed with cylindrical outer end portions exposed externally of said casing for supporting windings forming said pickup coils and the inner end portions thereof are provided with parallel, laterally spaced fins of rectangular outline spaced longitudinally of each side core piece and cylindrical base grooves positioned between said fins, said connecting core piece is of similar dimensions and form relative to said inner end portions of said side core pieces having parallel fins of rectangular outline spaced latterally along an axis paralleling the longitudinal axes of said side core pieces interspersed by cylindrical base grooves, and said casing is formed of complementary shell halves adapted to embrace said inner end portions of said side core pieces and said connecting core piece in laterally abutting relation to each other, said casing halves having inwardly projecting ribs complementing the lateral profile of said grooves adapted to seat in the grooves of said side core pieces and said connecting core piece to rigidly position the same in said casing.

4. A clamp-on transformer for picking up currents from an insulated conductor to monitor signals therein comprising a casing of insulating material, a magnetic circuit supported by said casing including a pair of laterally spaced elongated side core pieces disposed along generally parallel axes with inner end portions within said casing and outer end portions exposed externally of said casing and a connecting core piece within said casing forming a flux path between the inner end portions of the side core pieces and a bridging core member adapted to abut the ends of said outer end portions of said side core pieces, said side core pieces, connecting core piece and bridging core member forming a generally rectangular closed loop of magnetic core material, an elongated stem of core material fixed at one end to said bridging core member and extending into said casing through one of said side core pieces in slidably supported relation for axial reciprocation from an inner position wherein said bridging core member contacts both of said side core pieces to an outer position wherein the bridging core member is spaced from the end of the other of said side core pieces to accommodate passage of the insulated conductor therebetween, the magnetic core material forming said side core pieces, said connecting core piece, said bridging core member and said elongated stem being relay steel annealed in a hydrogen atmosphere having a non-linear field intensityflux density relationship, resilient means within said casing bearing against said stem for biasing the stem and bridging core member to said inner position, a push button slidably supported in said casing for manually forcing said stern and bridging core member against the biasing action of said resilient means to said outer position, pickup coils on the exposed outer end portions of said side core pieces disposed to be in inductive communication with currents in the insulated conductor when the conductor is positioned therebetween, and coupling means supported by said casing in electrically conductive communication with said coils for conducting to an external facility currents conductively coupled into the coils from the insulated conductor.

References Cited in the file of this patent FOREIGN PATENTS 822,570 Germany Nov. 26, 1951 

